1. Field of the Invention
The present invention relates to a state detecting method and an insulation resistance detector, in particular to a state detecting method for detecting a degradation of the insulation resistance between a ground and a direct current power supply, and a detector using the same.
2. Description of the Related Art
Conventionally, an insulation resistance detector using the above described state detecting method is known, for example, Japanese Patent Application Document No. 2005-114496. As shown in FIG. 1, the insulation resistance detector 50 includes a detecting resistor Rd connected in series to an insulating resistor Ri between a battery as the direct current power supply and a vehicle body, and a coupling capacitor mounted in between the insulating resistor Ri and the detecting resistor Rd for cutting off the direct current. The insulation resistance detector 50 further includes a pulse oscillation circuit 51 (pulse signal supplying member) for supplying a rectangular wave pulse signal P1 having a specific peak value with a series circuit consisting of the insulating resistor Ri, the coupling capacitor Co, and the detecting resistor Rd. Here, the peak value means the highest voltage in a pulse signal.
The pulse oscillation circuit 51, for example, includes a constant amplitude pulse generating circuit. When a control circuit 52 inputs a frequency signal S1 for the rectangular pulse signal P1 into the constant amplitude pulse generating circuit, a frequency of the rectangular wave pulse outputted from the constant amplitude pulse generating circuit is changed.
Further, a connection voltage Vx between the coupling capacitor Co-detecting resistor Rd is expressed by a formula (1) in which the peak voltage of the rectangular pulse signal P1 is divided by the detecting resistor Rd and the insulating resistor Ri.Vx=Vp*Ri/(Rd+Ri)  (1)
Where, Vp is a peak voltage of the rectangular pulse signal P1.
Accordingly, when the insulating resistor Ri is larger than the detecting resistor Rd as normal, the connection voltage Vx is nearly the same peak voltage as the rectangular pulse signal P1. On the other hand, when the insulating resistor Ri is reduced and the insulating resistor Ri is smaller than the detecting resistor Rd, the connection voltage Vx is reduced.
The insulation resistance detector 50 further includes a low pass filter 53 for outputting the connection voltage Vx after eliminating signals more than specific frequency. This low pass filter 53 is composed of a resistor Rf and a capacitor Cf and aimed for eliminating high frequency noises superimposed on the connection voltage Vx. An output of the low pass filter 53 is shaped at the waveform shaping circuit 54, then supplied to a control circuit 52. This control circuit 52 is composed of such as a microcomputer.
Next, a detecting principle of insulation resistance detector will be explained with reference to FIG. 5. In FIG. 5, L1 is a graph of frequency of the rectangular pulse signal P1 versus pulse peak value outputted from the low pass filter 53 indicating a normal state where the insulating resistor Ri is not reduced, and the insulation resistance detector 50 is not malfunctioning.
As shown in FIG. 5, in normal, when the frequency supplied from the pulse oscillation circuit 51 is less than 2.5 Hz, the output peak voltage of the low pass filter 53 is substantially equal to the peak voltage of the rectangular pulse signal P1 outputted from the pulse oscillation circuit 51. When the frequency supplied from the pulse oscillation circuit 51 is more than 2.5 Hz, as the frequency increases, the output peak voltage of the low pass filter 53 decreases.
This is because normally, when a time constant of the low pass filter 53 is large, when the frequency of the rectangular pulse signal P1 increases over the 2.5 Hz, the time between the rising time of the rectangular pulse signal P1 and the time for the output of the low pass filter 53 to reach 5 V as the peak value of the rectangular pulse signal P1 is shorter than the pulse width of the rectangular pulse signal P1. Namely, when the pulse width decreases corresponding to the increase of the frequency, before the output of the low pass filter 53 reaches 5 V as the peak voltage of the rectangular pulse signal P1, the supply of the rectangular pulse signal P1 is cut off, and the peak value is less than 5 V. When supplying the higher frequency of the rectangular pulse signal P1, the rectangular pulse signal P1 having the shorter pulse width is supplied, and the peak value of the low pass filter 53 is further reduced.
L2 in FIG. 5 shows a graph of the frequency of the rectangular pulse signal P1 versus the output peak voltage of the low pass filter 53, when the insulation resistance detector 50 is malfunctioning, for example, coupling capacitor Co or capacitor Cf is open.
As shown in FIG. 5, at the malfunction, even when the frequency increases, namely, when reducing the pulse width, the output peak voltage of the low pass filter 53 is substantially constant. This is because when the coupling capacitor or the capacitor Cf is open, the time constant of the low pass filter 53 decreases, and the rising time of the output of the low pass filter 53 is shorter than the normal state.
According to the above, when the coupling capacitor Co is open, namely, the connection between the insulation resistance detector 50 and the insulating resistor Ri is broken, the output peak voltage of the low pass filter 53 is constantly equal to the peak voltage of the rectangular pulse signal P1. Therefore, even when the insulating resistor Ri is reduced, the output of the low pass filter 53 is not reduced, and the reduction of the insulating resistor Ri cannot be detected.
Further, when the capacitor Cf of the low pass filter 53 is open, the noise elimination at the low pass filter 53 cannot be achieved, and the noise superimposed signal is inputted into the control circuit 52. In this case also, the reduction of the insulating resistor Ri cannot be detected correctly.
L3 in FIG. 5 shows a graph of the frequency of the rectangular pulse signal P1 versus the output peak voltage of the low pass filter 53, when the insulation resistance detector 50 is malfunctioning, for example, coupling capacitor Co or capacitor Cf is short.
As described above, when the coupling capacitor Co or capacitor Cf is short, then even applying the rectangular pulse signal P1, the output of the low pass filter 53 does not rise. Therefore, even when the frequency increases, namely, the pulse width decreases, the output voltage is constantly about 0.2 V.
As described above, when the coupling capacitor Co or capacitor Cf is short, the output of the low pass filter 53 is constantly low. Therefore, even when the insulating resistor Ri is not reduced, the output of the low pass filter 53 is reduced so that the reduction of the insulating resistor Ri cannot be detected.
Therefore, in a conventional detecting method, as shown in FIG. 5, when applying the rectangular pulse signal P1 of 2.5 Hz, and the output peak value of the low pass filter 53 is more than a threshold value X1, and when applying the rectangular pulse signal P1 of 5.5 Hz, and the output peak value of the low pass filter 53 is less than a threshold value X2, the insulation resistance detector 50 is judged as a normal state.
On the other hand, when the rectangular pulse signal P1 of 2.5 Hz and 5.5 Hz are applied, and the outputs of the low pass filter 53 are more than the threshold voltage X1, the insulation resistance detector 50 is judged as an open state.
Further, when the rectangular pulse signal P1 of 2.5 Hz and 5.5 Hz are applied, and the outputs of the low pass filter 53 are less than the threshold voltage X3, the insulation resistance detector 50 is judged as a short state.
Further, in Japanese Patent Application Document 2005-114496, the pulse width of the rectangular pulse signal P1 is changed by changing the frequency of the rectangular pulse signal P1. In Japanese Patent Application Document 2005-114497, a duty ratio of the rectangular pulse signal P1 is changed for changing the pulse width of the rectangular pulse signal P1.
Incidentally, in the insulation resistance detector 50, the output of the low pass filter 53 in response to the same frequency of the rectangular pulse signal P1 is different in each product. This is because a circuit constant of the insulation resistance detector 50, the voltage source, and circuit characteristics of the low pass filter 53 are varied in each product.
Namely, as shown by an alternate long and short dash line in FIG. 6A, there is a product of which output peak voltage of the low pass filter 53 is shifted up in response to the frequency of the rectangular pulse signal P1 against the other product shown in a solid line. As shown by an alternate long and short dash line in FIG. 6B, there is a product of which output peak voltage of the low pass filter 53 is shifted down in response to the frequency of the rectangular pulse signal P1 against the other product shown in a solid line.
However, according to the conventional state detecting method, on an assumption that the output peak voltage of the low pass filter 53 is constant in each product, and by comparing the output of the low pass filter 53 with the threshold voltages X1, X2, X3, normal, open, short states are detected.
Therefore, as shown by the alternate long and short dash line in FIG. 6A, when the output peak voltage of the low pass filter 53 is shifted up, even when the insulation resistance detector 50 is normal, the output peak voltage of the low pass filter 53 may be more than X2 in response to the 5.5 Hz rectangular pulse signal P1. Therefore, even when the insulation resistance detector 50 is normal, the output peak voltage of the low pass filter 53 may be more than X1 in response to 2.5 Hz rectangular pulse signal P1, and the output peak voltage of the low pass filter 53 may be less than X2 in response to 5.5 Hz rectangular pulse signal P1. Thus, the normal state of the insulation resistance detector 50 cannot be detected.
Further, as shown by the alternate long and short dash line in FIG. 6B, when the output peak voltage of the low pass filter 53 is shifted down, even when the insulation resistance detector 50 is open, the output peak voltage of the low pass filter 53 may be less than X1 in response to the 5.5 Hz rectangular pulse signal P1. Therefore, even when the insulation resistance detector 50 is open, the output peak voltage of the low pass filter 53 may be less than X1 in response to 2.5 Hz and 5.5 Hz rectangular pulse signal P1, and the open state of the insulation resistance detector 50 cannot be detected.
It is difficult to correctly detect the state of the insulation resistance detector 50 according to the comparison of the output peak voltage of the low pass filter 53 and the threshold voltages, because there is a shift up or shift down of the output peak voltage of the low pass filter 53. Namely, using the comparison of the output peak voltage of the low pass filter 53 with the threshold voltages X1, X2, X3 cannot correctly detect the state of the insulation resistance detector 50.
Further, according to the above, if the insulation resistance detector 50 is normal in response to the 2.5 Hz rectangular pulse signal P1, and then a short is occurred before the rectangular pulse signal P1 outputs 5.5 Hz pulse, the short state of the insulation resistance detector 50 cannot be detected. Namely, in a rare short that the short is intermittently occurred, a possibility of detecting the short state decreases.
Accordingly, an object of the present invention is to provide a state detecting method to detect a reduction of an insulating resistor correctly and easily, and to provide an insulation resistance detector using the state detecting method.